Developing new and useful materials, in the past, has been by prediction of the general chemistry of compositions and applying known testing methods to a small number of synthesized materials. Even with predicting and applying the currently known chemistry of materials, the number of materials that are predicted in a group is too large to properly analyze. The result of only analyzing a few materials in a predicted group leaves the great majority of predicted useful materials unexplored. Thus, the discovery and development of new materials have need for a method of synthesizing and analyzing new materials with a large number of variable compositions at a high throughput rate.
Combinatorial methods represent a new set of experimental tools that are well suited to explore systems comprised of a very large number of variable compositions. As a consequence of this characteristic, there has been a great deal of recent activity in the application of combinatorial synthesis to drug discovery, Chem. Rev. 1997, 97(2), where such large number of variable compositions are commonplace. In cases such as these, a great many different chemical structures need to be examined to find structural motifs, amino acid sequences (e.g. in bioactive polypeptides), or other molecular characteristics that exhibit the desired effect. The key to success in these efforts has been to exploit the power of combinatorial methods both for doing chemical reactions and for examining the efficacy of the resulting compounds, all in a parallel or high-speed serial fashion. The range of types of synthetic schemes and the systems to which they have been applied is typified by the articles in Chemical Reviews theme issue (Chem. Rev. 1997, 97(2)).
More recently, several groups have begun to apply combinatorial methods to materials problems. An example of this trend is the work being done at UC Berkeley by Schultz et al. To date, these groups have focused predominantly on materials properties, especially luminescence. Also, a recent report by Mallouk et al. points to the use of such methods in electrochemical applications (Science, 1998, 280, 1735). Specifically, Mallouk et al. used ink jet processing to deliver multiple metal complexes that served as electrocatalyst precursors to specific sites on a conductive substrate, employed chemical methods to reduce the complexes to produce metallic alloys and then used a novel fluorescence-based method to look for methanol oxidation activity. This appears to be one of the first uses of combinatorial methods in development of electrocatalysts. A particularly useful feature of this method was the demonstration of a parallel testing method. In addition to the efforts described above, several other groups have begun to explore the use of combinatorial methods for synthesis of materials with novel properties (Briceno et al. Science, 1995, 270, 273; Kobayashi et al. J. Am. Chem. Soc. 1996, 118, 8977).
An electrode's oxidation and reduction capabilities have led to the use of electrodes performing an essential step in synthesizing materials. One of the earliest description of using electrodes in combinatorial synthesis is by Fodor et al. (U.S. Pat. No. 5,424,186). Microelectrodes are used to remove protecting groups in the synthesis of organic molecules. Fodor et al. position an electrode over the protecting group to activate the desired deprotection step. Because of an electrode's versatility and control, the use of an array of electrodes in synthesis and analysis of materials is forthcoming.
In depositing materials onto an electrode many factors contribute to the composition of the material in the array. Some factors even affect the deposited materials in a solution after the material has already been deposited and other compositions are being deposited. In WO98/03521, Weinberg et al. express the need for homogeneous compositions of materials for a meaningful analysis of an array of materials. However, little work has been done to ensure that the array of materials may be analyzed for a desired characteristic and not for unwanted variations in morphology or composition.
An important feature of combinatorial synthesis is the ability to deposit meaningful compositions at discrete electrodes at a high rate of speed. In PCT WO98/14641, the complete disclosure of which is incorporated herein by reference for all purposes, McFarland et al. show an array of electrodes used for combinatorial synthesis and analysis, however, the use of changing out or adding components of the solutions in a solution bath results in an increased number of solutions when a hundred compositions are synthesized. Additionally, when more electrodes are employed to synthesize thousands or ten of thousands of compositions, the number of solutions or additions to solutions needed adversely affects the ability of high-throughput synthesis of compositions. McFarland et al., in WO98/14641, attempt to alleviate the need for a high number of solutions or additions to solution by using a variety of potentials at different electrodes to attempt to adjust the deposition of certain components in the solutions to vary the compositions at the electrodes electronically. While this method may result in a desired library of compositions, the compositions are affected by the method used to deposit and any meaningful analysis or screening is adversely affected by the morphology of the compositions. McFarland et al. discuss how multiple samples of varying composition can be prepared from solutions carrying various metal salts. However, they do not take the necessary steps to produce controllable morphology during the deposition or to maintain the composition of the samples after the deposition. For instance, when electrodepositing metals from solution at overpotentials that vary, a wide variety of surface morphologies are created. Those surface morphologies preclude easy and rapid comparison of the physical or chemical properties of the samples, specifically of the electrochemical, catalytic, or optical properties. Furthermore, when electroplating from solutions that contain Ni, Fe, Cu, and Zn, deposited samples that contain Zn, Ni, or Fe at their surfaces, the Zn, Ni, or Fe will react with the solution-bound salt of Cu to dissolve Zn, Ni, or Fe and deposit Cu. Similar reactions occur between Zn and Ni and between Zn and Fe. This general type of reaction occurs between any two species where the redox state of one species is at a less positive potential than the redox species of another species in the same environment. These reactions inadvertently change the surface compositions and morphologies of the deposits that have been prepared but remain in contact with the precursor-containing solution. Thus, controllable high-throughput synthesis and analysis of new materials using an array of electrodes is not yet feasible.
In order to synthesize and analyze a large number of new materials, a method of developing and analyzing new materials on an array of electrodes employing control techniques to ensure desired compositions and morphologies at known locations on the array is desirable.